1. Field of Invention
This invention relates to apparatus and method for mooring of marine structures. With more particularity, this invention relates to an anchor especially (although not exclusively) suitable for gravity installation, which embeds itself in a seabed by virtue of being dropped from a height above the seabed and being allowed to fall to the seabed of its own weight, and methods of installation of such anchor.
2. Description of Prior Art
A large suite of different anchor designs have evolved over the decades that rely on the anchor to embed or to be drug into the seabed to develop the holding capacity for the required mooring line load. These anchor designs vary widely in form or shape, from a caisson or pile to a conventional drag embedment anchor that includes flukes that open as the anchor is drug into location. These anchors have major disadvantages in that they are difficult to embed to deep penetrations below the seabed, where the underlying soils are stronger and thus can provide greater holding capacity.
Other anchor designs have been developed such as gravity installed anchors that are installed by freefalling under the force of gravity. These anchors are lowered down through the water column to a desired height above the seabed, and then released, whereby their own weight carries the anchor to and into the seabed under the influence of gravity. Examples of existing known example gravity installed anchors of this type include U.S. Pat. No. 6,106,199 to Medeiro, Jr. et al (Aug. 22, 2000) and U.S. Pat. No. 6,257,166 to Lieng (Jul. 10, 2001). The known prior art anchors are less effective that the present invention for several reasons. One disadvantage with the existing designs is that the behavior of the anchor under increasing loading does not allow the anchor to dig deeper into the stronger soils typically encountered at greater penetrations. The geometry and mooring line attachment point of the anchor of the present invention are designed such that the anchor will dig into the deeper soil under higher line loading. Another disadvantage of the prior art anchors involves limits placed on the mooring line direction (that is, loads in a direction radially outward from the anchor shaft) unless the attachment line is placed at the top of these anchors; such top placement severely limits the anchor's holding capacity. When using a side attachment of the mooring line on the prior art anchors, the mooring line angle is limited to a small variation from the fixed radial direction of the attachment point, so proper orientation of the anchor is critical to achieve an appropriate mooring spread. Such orientation (that is, orienting the attachment point in a desired radial direction) may be very difficult to accurately achieve. In contrast, the anchor of the present invention features a rotating mooring line attachment point on the shaft of the anchor, which is free to rotate to any direction around the longitudinal axis of the anchor shaft under influence of the mooring line force, thereby permitting the anchor to achieve optimum holding capacity while eliminating rotational orientation concerns during installation.